1. Field of the Invention
The present invention relates to a color image reading device, and more particularly to a color image reading device capable of precisely reading color image information on an original image, utilizing color separation means consisting of a one-dimensional blazed diffraction grating rotatable in the sub scanning direction and photosensor means including a line sensor, and adapted for use in a color scanner, a color facsimile or the like.
2. Related Background Art
There have already been proposed various devices in which color image information on an original image is focused through an optical system onto a line sensor (CCD), and is read in digital manner by the output signal from said line sensor.
As an example, FIG. 1 schematically illustrates the optical system of a conventional color image reading device. The light from a color image on an original 11 is condensed by an imaging lens 19, then separated by a prism 20 into three colors for example of red (R), green (G) and blue (B), and guided to respective line sensors 21, 22, 23 to be explained later. The color images focused on the line sensors 21, 22, 23 are line scanned in the sub scanning direction, whereby the image reading is conducted for each color.
Also FIG. 2 illustrates schematically the optical system of a conventional color image reading device. In this case the light from a color image on an original 11 is condensed by an imaging lens 29, and is guided through two color separating beam splitters 30, 31, each provided with a dichroic wavelength selecting transmissive film, thereby being separated into three lights corresponding to three colors.
Images of three colors are then focused on three line sensors on so-called monolithic 3-line sensor 32 in which said three line sensors are formed on a same substrate, and said images are line scanned in the sub scanning direction whereby the image reading is conducted for each color.
FIG. 3 is a schematic view of the monolithic 3-line sensor 32 shown in FIG. 2. In said sensor 32, three line sensors (CCD) 25, 26, 27 are formed in mutually parallel manner on a same substrate, with a certain distance therebetween, and each line sensor is provided thereon with an unrepresented color filter corresponding to the respective color of the image.
The distances S1, S2 of the line sensors 25, 26, 27 are generally in a range of 0.1 to 0.2 mm in consideration of various manufacturing conditions, and each pixel size W1, W2 of individual element 28 is selected, for example, as 7.times.7 .mu.m or 10.times.10 .mu.m.
The color image reading device shown in FIG. 1 requires three independent line sensors, and becomes complex and expensive in the configuration since there is required a 3P prims which necessitates a high precision and is difficult to manufacture. Also the adjustment at assembly is complicated since the alignment between the focused light and the line sensor has to be made independently for each color.
In the color image reading device shown in FIG. 2, the distance of the neighboring line sensors is 2.sqroot.2X wherein X is the thickness of the beam splitter 30 or 31. If the line distance preferred for manufacture is in the order of 0.1 to 0.2 mm, the thickness X of the beam splitters will be in the order of 35 to 70 .mu.m.
It is generally very difficult to construct a beam splitter of satisfactory optical flatness with such a small thickness, and the optical performance of the color image focused on the line sensor is inevitably deteriorated if the beam splitter of such small thickness is employed.
On the other hand, in a monolithic 3-line sensor as shown in FIG. 4, the distances S1, S2 of two lines 25, 27 from the central line 26 are generally selected mutually equal, and as an integral multiple of the pixel size W2 in the sub scanning direction, because of the following reasons.
In case of reading a color image with the above-mentioned monolithic 3-line color sensor in combination with an ordinary imaging optical system 45 as shown in FIG. 4, the three line sensors 25, 26, 27 simultaneously read three different positions 25', 26', 27' on the original image 11.
Stated differently, the signal components of three colors R, G, B on an arbitrary position on the original image 11 cannot be read at the same time, but have to be synchronized after reading with three line sensors and synthesized.
The synthesized signal of three color components can be relatively easily obtained by selecting the distances S1, S2 of the three line sensors as integral multiples of the pixel size W2, providing redundant line memories corresponding to said distances and delaying, for example, the G and R signals corresponding to the green and red lights with respect to the B signal corresponding to the blue light.
For this reason the distances S1, S2 of the line sensors 25, 27 from the central sensor 26 are selected as integral multiples of the pixel size in the sub scanning direction.
However, in such color image reading device, the use of plural line memories corresponding to the distances of three line sensors is extremely costly and leads to the complication of the entire device.
Also the conventional color image reading device has been associated with the drawback of high cost as it has employed three line sensors as the photosensor means.